Battery-less tags, by virtue of their potentially ultra-low cost and essentially unlimited shelf life, are important components for a broad class of important RFID applications. When an RFID inventory-tracking scheme requires every case or item within the purview of an inventory-control system to be tagged, which is the typical case for retail-distribution applications, battery-powered tags are generally considered cost-prohibitive, and battery-less tags are most often the only viable choice. When long-term storage of tagged items is involved, such as in a physical records archive managed with RFID technology, the finite shelf-life of batteries is an additional strong motivator for the use of battery-less tags.
Despite significant advances made in recent years in battery-less RFID tag technology, the adoption of this technology has significantly lagged the original expectations for RFID technology. An important impediment to more widespread adoption and utilization of battery-less RFID technology is the poor performance that is still frequently experienced when tags are on or near items that contain or comprise materials that interact strongly with RF propagation. Such materials include metal, dielectrics and lossy dielectrics that reflect, refract or attenuate RF energy incident on them or passing through them. Cans, foils, liquids, gels, dense powders, produce, meat and dairy products are just a few examples among numerous items that can severely impair the RF coupling between a reader and a tag.
Severe attenuation of a signal propagating from an RFID reader to a battery-less RFID tag is particularly problematic. The RF electromagnetic field strength required to operate a battery-less RFID tag is significantly higher than that required to communicate to an electronic receiver having an independent power supply such as a battery. Active electronic circuitry, powered by a battery or other power source, can indeed detect, decode and otherwise process extremely weak signals. A battery-less RFID tag, however, cannot operate such electronic circuitry until the tag has extracted sufficient energy from the RF electromagnetic field supplied by the reader or another external source. The incident RF field level required to provide operating power for the electronic circuitry is far greater than that required to communicate with already-powered circuits. The frequent difficulty in achieving the necessary incident RF field strength in the presence of material configurations with adverse RF propagation characteristics, while still satisfying regulatory constraints on radiated RF power levels, is an important technical obstacle currently preventing far more widespread adoption of battery-less RFID technology.
In prior applications of antenna arrays, there are typically only one or two degrees of freedom exploited, corresponding to elevation and azimuth angles for the far-field radiation pattern. In relatively rare applications, multiple beams might be formed, or radiation might be focused at a finite distance rather than at infinity, whereas far-field patterns are essentially “focused at infinity”. Even in such relatively exotic applications, however, the degrees of freedom utilized are far less than the total degrees of freedom inherently available with independent control of individual antenna elements.
Prior applications of array technology are characterized by one or more of the following:                The medium is homogeneous, such as free space, or sufficiently close to homogeneous such that a homogeneous medium is assumed for the control of the array;        The medium differs from a homogeneous one by a constant, known factor, such as a protective radome, a supporting structure that interacts with the array, a ground plane or approximate ground plane, or a nearby half-space filled with a different homogeneous or approximately homogeneous medium;        The medium is sufficiently inhomogeneous to affect the propagation in a potentially adverse way, as exemplified by an environment containing walls, trees or other structures, but the antenna system makes no adjustments specific to the particular configuration of this surrounding material, other than possibly an adjustment in its angular sensitivity, e.g., pointing direction;        The array does make adjustments that mitigate the effects of adverse propagation characteristics such as multipath, but requires the presence of a signal originating from the intended focal point in order to adapt the array settings, as is the case for a “rake receiver;”        The array makes adjustments that peak its response to a signal emanating from an unknown location, but requires the presence of a signal emanating from that location, and particular to it (different in some way from similar signals that may be emanating from other locations), in order to adapt the array settings, as in the case of adapting the array to a transponder or modulator with a self-contained power source;        The array makes adjustments that mitigate for unwanted signals, in which case there is by definition a signal originating from the location or direction of the intended null in the array antenna pattern.        